1. Field of the Invention
The present invention relates to a switching regulator for outputting a constant voltage, and more particularly, to an overload protection circuit for interrupting current supply to an output to protect a circuit when an overcurrent flows to an output terminal.
2. Description of the Related Art
Switching regulators are used as voltage supply sources of various electronic device circuits. The switching regulator functions so as to output a constant voltage to an output terminal regardless of voltage fluctuations of an input terminal. In addition, the switching regulator also has an important function of overload protection performed by interrupting current supply for protection of a circuit when a current to be supplied from the output terminal to a load dramatically increases and exceeds the maximum current.
FIG. 5 is a block diagram of a switching regulator control circuit including an overload protection circuit.
The conventional switching regulator control circuit includes a triangle wave oscillator 1, an error amplifier 2, a PWM comparator 3, an error amplifier output detection circuit 4, a timer circuit 5, a gate circuit 6, a reference voltage circuit 7, and a buffer circuit 8.
The reference voltage circuit 7 outputs a reference voltage Vref. The triangle wave oscillator 1 outputs a triangle wave Vramp oscillating between an upper-limit level VH and a lower-limit level VL. The error amplifier 2 compares a feedback voltage Vfb of an output voltage Vout of the switching regulator and the reference voltage Vref, and amplifies a difference voltage. The PWM comparator 3 compares an output voltage Verr of the error amplifier 2 and the triangle wave Vramp, and outputs a PWM signal Vpwm. The gate circuit 6 controls the output of the PWM signal Vpwm. The buffer circuit 8 power-amplifies the output of the gate circuit 6, and outputs the amplified signal to a driver transistor (not shown). The error amplifier output detection circuit 4 monitors the output voltage Verr of the error amplifier 2. The timer circuit 5 starts its counting operation based on output results of the error amplifier output detection circuit 4.
Here, the error amplifier output detection circuit 4 includes a reference voltage circuit for outputting a reference voltage Vref2 (>VH), and a comparator for comparing the output voltage Verr of the error amplifier 2 and the reference voltage Vref2. When Verr>Vref2 is satisfied, the comparator outputs an overload state detecting signal to the timer circuit 5. With this, the timer circuit 5 starts its counting operation, and after a predetermined time period has elapsed, the overload state detecting signal is output to the gate circuit 6. In response to this, the gate circuit 6 controls the driver transistor so as to be non-conductive. In this manner, the output voltage Vout drops to 0 V, and the difference voltage between the reference voltage Vref and the feedback voltage Vfb increases, to thereby maintain the relationship of Verr>Vref2. Thus, the driver transistor continuously maintains the non-conductive state (see, for example, Japanese Patent Application Laid-open No. 03-52556). FIG. 6 illustrates a timing chart of an operation of the conventional switching regulator control circuit.
As described above, the overload protection circuit of the conventional switching regulator control circuit prevents thermal breakage of an output transistor due to the overload.
However, in a case of a relatively small overload, the change amount of the output voltage Verr of the error amplifier 2 is small, and hence the output voltage Verr does not exceed the reference voltage Vref2 in some cases. Therefore, it is demanded to lower a set value of the reference voltage Vref2 to detect the overload with a smaller output current Iout. In view of this, the set value of the reference voltage Vref2 is set to a voltage (VH>) slightly larger than Verr(max).
However, in order to obtain a reference voltage Vref2′ slightly larger than Verr(max), adjusting means for correcting manufacturing fluctuations is necessary because the manufacturing fluctuations of the upper-limit level VH and the lower-limit level VL of the triangle wave Vramp cause a serious trouble. Further, in order to correct the manufacturing fluctuations, it is also necessary to measure the upper-limit level VH and the lower-limit level VL of the triangle wave Vramp corresponding to a voltage at an internal node of an integrated circuit (IC).